Frailty has been theoretically defined as a clinically recognizable state of increased vulnerability in older adults. Although numerous instruments have been developed to identify frailty, consensus on best measures has not been achieved. The two most commonly cited frailty measures in the geriatric literature are the Physical Frailty Phenotype (PFP) and the Frailty Index (FI). The number of citations of the FI and the PFP has quintupled and octupled respectively in last five years. With the growing popularity of these instruments simplifications and approximations of existing phenotypes have proliferated, typically motivated by either availability of measures in specific studies or the demand for speed and ease of patient evaluation in clinical practice. The idea of tailoring measures to specific research or clinical settings may be appealing. However, there also are potential harms if the instruments measure fundamentally different aspects of older adults' health, or if they are differentially effective in accomplishing different aims of frailty ascertainment (e.g., frailty syndrome characterization vs. risk stratification). Although a number of epidemiological studies have been conducted to compare the different frailty assessment tools, the comparisons so far have almost exclusively focused on predictive validity. Using data collected over 9 years in the 5888 older adults age 65 and older from the Cardiovascular Health Study, this study conducts the comparative analysis in a novel direction to: systematically evaluate possible simplifications of the PFP criteria that could be used for (1) their internal validity regarding frailty syndrome identification, (2) accuracy in discriminating the risk of frail and non-frail persons for adverse outcomes of aging, (3) usefulness as a potential marker of increased vulnerability, as well as (4) determine the level, sources, and predictors of the discordance between the PF and the FI in frailty classification. By elucidating the values and limits of the two most commonly used frailty instruments and assessing potential simplifications of the PFP, the proposed study promises to provide much needed guidance on the selection of frailty measures to match clinical aims of frailty assessment, with the ultimate goal of identifying new arenas for frailty prevention and treatment.